The present invention relates primarily to an optical transmission module used for an optical transmission system or optical conversion system (these systems will be called commonly optical communication systems), and also relates to the technique of optical coupling between a light emitting or receiving device and an optical fiber, between a light emitting or receiving device and an optical circuit and between an optical circuit and an optical fiber in an optical transmission module.
In the progressive evolution of information transmission paths based on the optical scheme, information transmission through optical fiber cables is becoming prevalent not only among industrial buildings, but also among apartment buildings and individual houses. One of the crucial affairs to be treated at present is obviously the price reduction of the optical transmission system, particularly the price reduction of the optical transmission module which is connected to the communication apparatus of each subscriber.
It has been a common design for the optical coupling between an optical device, such as a semiconductor laser source, and an optical fiber cable or optical waveguide to place an optical lens between these parts so as to enhance their optical coupling efficiency. However, the scheme of placing a lens between the optical device and the optical waveguide not only results in an increased number of component parts, but it compels the worker to make alignment of these separate parts. This extremely delicate work has been a major barrier to the price reduction of the optical transmission module which is installed on the part of the subscriber.
As a scheme of overcoming this problem, there has been devised a semiconductor laser source integrated with a beam spot size converter as described, for example, in Japanese Patent Unexamined Publication No.hei-5-249331, Japanese Patent Unexamined Publication No.2000-214340, and the device has been developed in recent years so as to be put into practice.
First, the beam spot size converter will be explained with reference to FIG. 10. The figure shows schematically an optical coupling system between a semiconductor laser source and an optical waveguide, and is also used to explain the evaluation of the light beam coupling efficiency.
The light beam is assumed here to be a Gaussian beam, and the coupling of Gaussian beams of the 0-th order will be dealt here, since the light beam used in the optical communication has a single guided mode in most cases. For beam spot sizes W1 and W2 of a semiconductor laser source 23 and optical waveguide 22, respectively, (the beam spot size is the radius of beam spot at which the amplitude of Gaussian beam falls to 1/e of the center value) at the respective beam waists (the beam waist is the position of beam axis where the curvature of the Gaussian beam wave surface is infinity), a distance Z between the beam waists, a beam deviation (mis-alignment) X in the direction which is vertical from the optical axis, and a light beam wavelength xcex, the coupling efficiency xcex7 is expressed by the following formula.                     η        =                  κexp          ⁢                      {                                          -                κ                            ⁢                                                                    xe2x80x83                                    ⁢                                      x                    2                                                  2                            ⁢                              (                                                      1                                          w                      1                      2                                                        +                                      1                                          w                      2                      2                                                                      )                                      }                                              (        1        )            
where xcexa is given as:                     κ        =                  4                                                    (                                                                            w                      1                                                              w                      2                                                        +                                                            w                      2                                                              w                      1                                                                      )                            2                        +                                          (                                                      λ                    ⁢                                          xe2x80x83                                        ⁢                    z                                                        π                    ⁢                                          xe2x80x83                                        ⁢                                          w                      1                                        ⁢                                          w                      2                                                                      )                            2                                                          (        2        )            
The above formulas reveal that the coupling efficiency and the tolerance against the beam mis-alignment are improved by: (1) making W1=W2, and (2) making this value as large as possible.
The conventional semiconductor laser source 23 has a very small beam spot size W1, i.e., incident beam spot size W1, as compared with the beam spot size W2 of the optical fiber or optical waveguide 22, and therefore the condition W1=W2 is not met and thus the coupling efficiency is not good enough.
A semiconductor laser source initegrated with a beam spot size converter is intended to increase the beam spot size W1 close to W2, thereby improving the coupling efficiency and tolerance in consequence.
A laser source integrated with a beam spot size converter is manufactured based on the selective crystal growing process so as to make a tapered film thickness at the emission port of the core section. The integrated structure of the beam spot size converter influences the optimal design of the laser source itself, and it also causes the machining error to be highly influential on the laser characteristics. On this account, this laser source suffers a poor yield as compared with the conventional laser source, and consequently the costly laser source does not contribute to the price reduction of the optical transmission module.
There is a limit in the expansion of beam spot size achieved by the tapered film structure based on the selective crystal growing process, and the beam spot size attainable at present is around 10 degrees in terms of the divergence angle of far field pattern for a light beam approximated to be a Gaussian beam. There is still a significant difference of it from about 5 degrees of the divergence angle of the optical fiber. Therefore, for increasing the coupling efficiency and facilitating the parts assembly, even in the case of using a laser source with a beam spot size converter, it is necessary to develop a new optical coupling technique to be applied to the laser source.
Although the conventional technique of placing an optical lens between the semiconductor laser source and the optical waveguide is a conceivable scheme, increased component parts and intricate assembling work resulting from the adoption of this scheme precludes the accomplishment of cost reduction of the optical transmission module.
Accordingly, a first object of the present invention is to provide an optical transmission module and an optical communication system which have the improved coupling efficiency between the optical component parts.
A second object of the present invention is to provide an optical transmission module and an optical communication system which have at least one of the improved coupling efficiency between the optical component parts or the improved tolerance.
In order to achieve the first objective, the inventive optical transmission module includes a first optical waveguide which expands the spot size of a light beam along the light propagating direction and a second optical waveguide which reduces the beam spot size, which has been expanded by the first optical waveguide, along the light propagating direction, with at least one of the first and second optical waveguides having its refractive index of its core section varied along the x axis which intersects the z axis of the light propagating direction vertically on a plane perpendicular to the z axis or along the y axis which intersects the z axis horizontally on the plane.
The waveguide core section is formed of a first material having refractive index n1 in its central portion and a second material having refractive index n2 which is smaller than n1 in its portions on the upper and lower sides or on the right and left sides of the portion of the first material.
The waveguide core section is formed by sequential lamination of a first core having refractive index n1,., an (nxe2x88x921)th core having refractive index nnxe2x88x921, and an n-th core having refractive index nn, with these cores having a relation in terms of their values of refractive index of: n(n+1)/2 greater than n(nxe2x88x921)/2 greater than . . .  greater than n2 greater than n1 and n(n+1)/2 greater than n(n+3)/2 greater than . . .  greater than nnxe2x88x921 greater than nn.
A number of cores are aligned in the z-axis direction to form the core section.
The cores have their widths in one of the x-axis or y-axis direction varied in the z-axis direction.
In order to achieve the second objective, the inventive optical transmission module includes a first optical waveguide which expands the spot size of a light beam along the light propagating direction, a second optical waveguide which retains the beam spot size which has been expanded by the first waveguide, and a third optical waveguide which reduces the beam spot size which has been retained by the second waveguide, with at least one of the first, second and third waveguides having its refractive index of its core section varied along the x axis which intersects the z axis of the light propagating direction vertically on a plane perpendicular to the z axis or along the y axis which intersects the z axis horizontally on the plane.
The waveguide core section is formed of a first material having refractive index n1 in its central portion and a second material having refractive index n2 which is smaller than n1 in its portions on the upper and lower sides or on the right and left sides of the portion of the first material.
The waveguide core section is formed by sequential lamination of a first core having refractive index n1, . . . , an (nxe2x88x921)th core having refractive index nnxe2x88x921, and an n-th core having refractive index nn, with these cores having a relation in terms of their values of refractive index of: n(n+1)/2 greater than n(nxe2x88x921)/2 greater than . . .  greater than n2 greater than n1 and n(n+1)/2 greater than n(n+3)/2 greater than . . greater than nnxe2x88x921 greater than nn.
A number of cores are aligned in the z-axis direction to form the core section.
At least one core of the core section has a cross-sectional shape on the y-z plane of a circle, ellipse, approximate circle or appropriate ellipse.
The cores have their widths in one of the x-axis or y-axis direction varied in the z-axis direction.
In the foregoing structure of the optical transmission module, clad layers, which are lower in refractive index than the cores, are formed in spaces between cores aligning in the z-axis direction.